The invention relates to a method for operating a particle sensor, wherein the particle sensor has, on its surface, at least two inter-digital electrodes which engage one in the other and to which a sensor voltage U(IDE) is at least temporarily applied in order to determine loading of the particle sensor with soot particles, and a sensor current I(IDE) across the electrodes is measured and evaluated, wherein, in order to remove the loading with soot, a heating element can be additionally provided, with which heating element the particle sensor is heated in a regeneration phase.
The invention also relates to a corresponding device for carrying out the method according to the invention, wherein the particle sensor is connected to an engine controller or a sensor control unit, and the engine controller or the sensor control unit has devices for diagnosing the loading of the particle sensor with soot.
Particle sensors are used today, for example, for monitoring the emission of soot by internal combustion engines and for on-board diagnostics (OBD), for example for functional monitoring of particle filters. In this context, collective, resistive particle sensors are known which evaluate a change in the electrical properties of an inter-digital electrode structure on the basis of accumulations of particles. Two or more electrodes may be provided which preferably engage one in the other in a comb-like fashion. The electrodes are short-circuited by an increasing number of particles which accumulate on the particle sensor, and this can result in an electrical resistance which decreases as the accumulation of particles increases, a decreasing impedance or in a change in a characteristic variable, such as a voltage and/or a current, which is associated with the resistance or the impedance. For the purposes of evaluation, a threshold value, for example of a measuring current between the electrodes, is generally defined, and the time until the threshold value is reached is used as a measure of the accumulated quantity of particles. Alternatively, a rate of change of the signals can also be evaluated during the accumulation of particles. If the particle sensor is fully loaded, the accumulated particles are burnt off in a regeneration phase using a heating element which is integrated in the particle sensor.
Such a resistive particle sensor is described in DE 101 33 384 A1. The particle sensor is constructed from two comb-like electrodes which engage one in the other and which are at least partially covered by a capturing sleeve. If particles from a gas stream become deposited on the particle sensor, this leads to a change in the impedance of the particle sensor which can be evaluated and from which the quantity of accumulated particles, and therefore the quantity of particles carried along in the exhaust gas, can be inferred.
DE 101 49 333 A1 describes a sensor device for measuring the moisture of gases, with a resistance measuring structure which is arranged on a substrate, wherein the measuring structure interacts with a layer of soot, and a temperature measuring device is provided. This sensor device can also be used to determine the concentration of soot in the exhaust gas of an internal combustion engine.
DE 10 2004 028 997 A1 discloses a method for controlling the accumulation of particles on a sensor element which has a first electrode and a further electrode and to which a first voltage U1 and a second voltage U2 can be applied at voltage terminals. There is provision here that the sensor element can be operated with a raised voltage U1 during a first time period t1, and, after a triggering threshold AP of the sensor element has been exceeded, said sensor element can be operated with a lower voltage U2, which is lower than the raised voltage U1. The method permits the time after regeneration of the sensor element in which no measuring signal is available until the time at which a signal which can be evaluated is obtained as a result of accumulation of a sufficient quantity of particles to be shortened by virtue of the fact that the sensor element is operated with a raised operating voltage during this phase. The raised operating voltage brings about an increased rate of accumulation of particles on the sensor element. If a sufficiently large quantity of particles has accumulated on the sensor element, so that a measuring signal which can be used is present, the sensor element is operated with a lower voltage with a correspondingly lower rate of accumulation of particles, with the result that the measuring period up to the next necessary regeneration of the sensor element is lengthened. The method accordingly provides two successive operating phases, a first phase with a raised operating voltage during which there is still no adequate measuring signal present, and a second phase with a reduced voltage during which the actual measurement of the particle concentration takes place. In this context, the resistance or the impedance of the sensor element is determined during both phases by means of a corresponding current measurement, on the one hand for detecting the triggering threshold and on the other hand for determining the rate of accumulation of particles. In both phases, a defined accumulation of particles is necessary. The selected voltages in both phases accordingly constitute a compromise between optimized accumulation of particles and precise measurement of resistance or impedance.
DE 103 19 664 A1 discloses a sensor for detecting particles in a gas stream, in particular particles of soot in an exhaust gas stream, having at least two measuring electrodes which are arranged on a substrate made of an electrically insulating material. In this context there is provision that the measuring electrodes are coated by a protective layer. The protective layer protects the electrodes against corrosion under extreme environmental conditions. In this context, the protective layer can be embodied as an electrical conductor or as an electrical insulator. A conductive protective layer permits the particle concentration to be determined through resistance direct current measurement, in which case a parallel connection between the electrodes is produced by means of the protective layer and the accumulated particles. In the case of an insulating protective layer, it is necessary to measure the impedance using an alternating voltage.
In order to regenerate the particle sensor after particles have accumulated, the sensor element is burnt clean using an integrated heating element. This must be carried out at certain time intervals in order to avoid falsification in the determination of the particle concentration.
The start of regeneration is usually triggered by the current across the sensor element, the so-called IDE current or sensor current I(IDE), exceeding a defined threshold value current. The concentration of soot in the exhaust gas can be inferred from the time until the threshold value current is reached.
In order to be able to obtain a high level of sensor accuracy, it is decisive to be able to determine as accurately as possible the time when the defined evaluation threshold is reached. Since the threshold currents are typically in the region of several μA, the signal can easily be falsified by shunts, for example owing to moisture condensing on the sensor. The separation of these shunt currents from the searched-for IDE current has hitherto presented a challenge for the development of sensors.